What is alfalfa meal, and is it a good fertiliser?

Ingredients

What is alfalfa meal, and is it a good fertiliser?

By Joe, Founder of Dr Forest · Published May 2026 · Edited 19 Sep 2026

Alfalfa meal is milled lucerne sold as a slow organic nitrogen source — useful once cold UK soil has warmed. It also turns up as a soil improver, as horse fodder, and in rose society lore as a growth hormone in a bag. Only the nutrition story survives contact with the literature.

Alfalfa meal is the dried, milled top growth of lucerne, Medicago sativa, sold as a slow-release organic fertiliser. Compositional data put it at roughly 2.9% nitrogen, 3.1% K₂O and 3.1% CaO, with modest phosphorus and very little magnesium. Its carbon-to-nitrogen ratio of 12 to 15 sits just below the point where soil microbes start releasing nitrogen rather than locking it away, so it feeds over weeks rather than days.

That is the honest case for alfalfa, and it is a good one. The other case, the one that shifts most of the bags, is that alfalfa contains a plant growth hormone. That claim needs unpicking, and it gets it further down this page.

What alfalfa meal actually is

Lucerne is a deep-rooting perennial legume grown mostly as forage. Cut, dried and milled, it becomes alfalfa meal. Pressed through a die, it becomes alfalfa pellets. Same material, different physical form. Alfalfa is the American name and lucerne the older British one, and UK suppliers use both, sometimes on the same sack.

Because it is a legume it fixes nitrogen in root nodules whilst it grows, and because it roots deeply it draws minerals from further down the profile than most annual crops. Neither fact does much for you once the stuff is dried and bagged. What matters then is what the plant contains and how quickly soil organisms take it apart.

What is in a bag of it

The best compositional data on dried alfalfa does not come from the garden trade. It comes from animal nutrition, where dehydrated lucerne has been analysed tens of thousands of times. Feedipedia, the INRAE, CIRAD, AFZ and FAO feed database, gives a mean crude protein of 18.3% of dry matter across 14,989 samples, which at the standard 6.25 factor is about 2.9% nitrogen. Calcium averages 22.1 g/kg of dry matter, potassium 25.6 g/kg and magnesium 2.1 g/kg.

Two things stand out. Potassium is higher than most alfalfa labels declare, ours included. Label figures are conservative minimums rather than a description of the material, and batch variation is real: crude protein across those samples runs from 13.1% to 27.9%, driven by growth stage, cut number and how much leaf survives handling relative to stem.

Phosphorus is the quiet one, and it is worth defending. At about 0.6% P₂O₅ alfalfa is not a phosphorus fertiliser, but it is not empty of it either, and the modest figure is a kindness rather than a shortcoming. Phosphorus accumulates in soil. A garden that has had manure or a phosphate feed on it for years is usually well supplied, and piling more on achieves nothing except what washes into the watercourse. Alfalfa gives you a small, steady amount alongside everything else, which is what an established bed actually wants.

It carries trace elements too, which a straight nitrogen feed does not. Manganese runs at about 32 mg per kilogram of dry matter, zinc at 30 and copper at 6. Iron is higher again, though that figure rests on only 29 samples and ranges from 223 to over 1,000 mg/kg, so take it as present rather than precise. These are small numbers, but micronutrient shortages in gardens are shortages of milligrams, and a bulky material spread at 100 to 200 g per square metre delivers them slowly and in company rather than as a corrective spike.

The second is the magnesium. At roughly one part magnesium to twelve parts potassium, alfalfa is a poor magnesium source in the company of a generous potassium one. On ground that is already short of magnesium, repeated alfalfa will not help and may sharpen the symptoms. If your older leaves are yellowing between the veins, pair it with kieserite or Yorkshire polyhalite rather than adding more alfalfa.

Meal or pellets, and which acts faster

A pellet is ground alfalfa compressed through a die. It is not a different material and it is not a coated slow-release granule. The compression is there for handling, which matters more than people expect, because once a pellet takes up water it slakes back into powder quickly.

So the pellet is only slower whilst it stays dry. Scatter pellets over a dry bed in April and they will sit on the surface looking exactly as they did on the day, doing very little. Wet them and they collapse, and from that point they behave much as the meal does.

That gives you an instruction rather than a trade-off. If you are using pellets and you want the quicker release, water after application. If you would rather it went slowly, leave it to the weather and let the first proper rain start the clock.

Pellets are easier to live with either way. They scatter evenly, they do not blow about, they do not coat your hands, and they store without compacting into a block.

How the nitrogen arrives

Alfalfa's carbon-to-nitrogen ratio is 12 to 15. That number tells you more than the NPK does.

When plant material goes into soil, the microbes breaking it down either release its nitrogen or hold on to it, depending on how much carbon they have to process per unit of nitrogen. Justes, Mary and Nicolardot incubated crop residues for 168 days and found the switch sits around a C:N of 13. Below that, net nitrogen release throughout. Between 13 and 20, a few weeks of immobilisation first, then release. Above 26, it stays locked up for the duration.

Alfalfa straddles that line. A leafy, high-protein batch releases from the start. A stemmier batch may take a fortnight to turn positive. Perennia's review of organic nitrogen sources notes that at least one study showed net immobilisation from alfalfa meal, and adds that this most likely varies with the nitrogen content of different meals. Even the most obliging residues in the Justes work gave up only about a fifth of their nitrogen in the first week.

So alfalfa is not a rescue feed. If a plant is hungry now, it needs something soluble now.

All of that assumes a warm soil, and here the American guidance stops being much use to us. Nitrogen mineralisation is steeply temperature-dependent. Miller and Geisseler incubated 55 agricultural soils at 5, 15 and 25°C and found a mean Q₁₀ of 2.87, meaning the rate nearly trebles for every ten degrees. It did not shift with organic matter content, texture, salinity or pH, which makes it an unusually portable rule of thumb.

Perennia's fact sheet puts the effect of a ten-degree drop on alfalfa pellets at nearer a fifth, which is far gentler. That figure is alfalfa-specific whilst the Q₁₀ is measured across many more soils and peer reviewed, so I lean on the Q₁₀. The answer is probably between the two. The direction is not in doubt either way.

Work that through and the consequence for a British garden is blunt. A bed at 8°C is releasing nitrogen at under half the rate the same bed manages at 15°C. There is no sense feeding cold ground and expecting a spring response, and nitrogen mineralised before anything is taking it up is nitrogen at risk of washing out of a wet British spring. Wait for the soil to warm, and judge the timing by what the soil is doing rather than by the calendar. A cheap soil thermometer will tell you more about when your feeding is going to work than any packet will.

The carbon that stays behind

Most nitrogen fertilisers are nutrient and nothing else. The plant takes what it needs, and there is nothing left in the ground to show for it. Alfalfa is a different proposition, and this is the part of the case that gets the least attention.

Somewhere between 35 and 44 per cent of alfalfa meal by dry weight is carbon. That falls straight out of the ratio: 12 to 15 parts carbon to every part of nitrogen, on about 2.9% nitrogen. Nearly 29% of the dry matter is crude fibre, which is the cellulose, hemicellulose and lignin that held the plant up in the field.

Nitrogen leaves in weeks. Carbon does not. The fibrous fraction is taken apart slowly, feeds soil organisms on the way through, and a share of it ends up as the stable organic matter that holds water and structure together. At bag rates you are mainly buying a fertiliser that also leaves some organic carbon behind — useful, but not a substitute for compost as a structure builder, and long-term SOM gains from meal alone are unproven.

The ratio is what makes that work rather than backfire.

Straw or woodchip brings far more carbon, but at a ratio in the sixties or the hundreds the microbes taking it apart have to scavenge nitrogen out of the soil to do the job, and your plants go short whilst they do it. A soluble feed has no carbon in it at all. Alfalfa sits at the point where there is just enough nitrogen to pay for its own decomposition, with the carbon left over as profit.

The growth hormone question

Every alfalfa product sold to gardeners gets round to triacontanol eventually. Here is the whole story, as far as the literature goes.

Triacontanol is a thirty-carbon primary alcohol found in plant leaf wax. Chibnall and colleagues first isolated it from lucerne wax in 1933. In 1977, Ries and co-workers reported in Science that it acted as a plant growth regulator, and later work puts the active concentration around 10 µg per litre, or 2.3 × 10⁻⁸ M. That is parts per billion. The compound is potent, and the mechanism has been studied properly since.

What happened next is the part nobody quotes. The standard review of the field, by Naeem, Khan and Moinuddin, reports that "the results of 46 field experiments, conducted in several parts of the world, generally showed no significant increases in crop yield, except one study conducted in Japan". The same review notes, in the very next sentence, that foliar and seed treatment at 0.05 and 0.10 mg per litre raised cotton yield by 12% and 31%. So the position is not that triacontanol does nothing. It is that a compound which behaves beautifully in a growth chamber has been very hard to make work in a field.

There is a specific reason for that, and it bears directly on whether a bag of dried leaf can deliver anything. Triacontanol's activity is knocked out by other long-chain alcohols sitting next to it. Jones, Wert and Ries found that 1-octacosanol inhibited the rice response to triacontanol at concentrations as low as 2.4 × 10⁻¹² M, roughly a ten-thousandth of the active dose. Ries and colleagues then showed how sharp the purity requirement is.

A compound that loses half its effect to three percentage points of impurity is not a compound you deliver in a bag of dried leaf.

Alfalfa leaf wax is precisely a mixture of C28, C29 and C30 alcohols. Whatever triacontanol sits in the meal arrives packaged with its own antagonists, and at ratios far worse than the 96.4% preparation that already halved the response.

Two further problems. Triacontanol is practically insoluble in water and only works when formulated as a colloidal dispersion of sub-micron crystals; in meal it is intact wax bound up in dried leaf tissue, which has never been melted or dispersed. And when Eriksen, Haugstad and Nilsen compared the routes directly, a foliar spray raised tomato yield significantly whilst adding it to the growth medium gave only a temporary increase, with maize showing nothing by either route. Soil is the weak route.

Nobody has published a measurement of triacontanol in commercial alfalfa meal. Nobody has run alfalfa meal against a nutrient-matched control to separate a hormone effect from the nitrogen it delivers. Until someone does, every garden response anyone has reported is fully explained by feeding the plant. The most recent peer-reviewed review of alfalfa in horticulture, published this year, does not mention triacontanol at all.

None of which makes triacontanol useless. It makes the delivery the whole question. Where the compound has worked, it has worked as a purified foliar spray at parts per billion, which is how it goes into Brix+, and not as dried leaf scattered on soil. If triacontanol is what you are after, that is the route to take. Alfalfa is the wrong vehicle for it, whatever the bag says.

How much to use, and when

Rates in grams throughout, because a handful is not a unit and pellets and meal do not weigh the same for a given scoop. Dr Forest sells alfalfa pellets for the garden — same dried lucerne as meal, pressed so they stay put when wet.

Apply to moist soil, scratch it into the surface, water it in. Keep it 10 cm clear of stems and off the graft union on roses. The planting-hole figure is mixed through the whole volume of backfill, not tipped into the bottom of the hole, and it should not sit against bare roots.

For context on the rose figures, the RHS feeds border roses twice a year, in March or April and again in mid-summer after the first flush, at 70 g per square metre of a general-purpose or rose fertiliser. Alfalfa is weaker than a compound rose feed, which is why the per-bush figures above are higher. It is a soil amendment doing a feeding job, not a concentrate.

There is no useful way to overdose a plant with alfalfa in the way you can with a soluble feed, since the nitrogen has to be mineralised before it goes anywhere. The thing to watch on repeat application is the calcium. Alfalfa runs about 2.2% calcium and around 12% ash, so year after year it adds lime-adjacent material to ground that in much of Britain has plenty already.

Where it fits, and where it does not

Alfalfa suits hungry, leafy things and repeat-flowering shrubs: brassicas, courgettes and squash, sweetcorn, roses, clematis, dahlias, and anything going into a bed that gets emptied and refilled every year. It suits a living soil approach, where you are feeding the system rather than the plant. It will feed a lawn too, though at 2.5% nitrogen it is mild for the hungriest thing in the garden and does better there as a soil builder than as the whole feeding programme.

It is a poor choice on ericaceous beds, given the calcium load. It is a poor choice as an emergency feed. And it is worth a moment's thought before sowing into freshly amended ground.

Alfalfa carries phenolic acids and saponins, and two separate effects get muddled together here. Autotoxicity is alfalfa suppressing alfalfa, which is why farmers leave a year between an old stand and a new sowing. Allelopathy is alfalfa suppressing something else, and that is documented too. Powdered alfalfa root at 0.25% by weight destroyed the root systems of winter wheat seedlings in Oleszek and Jurzysta's work, and Ghimire and colleagues found alfalfa extracts inhibited callus growth in five unrelated species. Both used fresh root material or concentrated extracts rather than dried meal at garden rates, so the read-across to a bagged fertiliser is not established. Baisley and Cassida's 2025 review makes the same point from the other side: the laboratory bioassays show the effect consistently, the field-scale demonstrations do not.

The same Oleszek and Jurzysta work found the toxicity fades within days as soil microbes hydrolyse the glycosides, and faster in heavier soils than in loose sand. So the practical answer is unfussy. Work the meal in, leave a fortnight before direct-sowing fine seed, and a little longer on light sandy ground. Transplanting established plants needs no wait at all.

On the compost heap, alfalfa works for the same reason grass clippings do. It brings nitrogen at a low C:N ratio into a heap that is usually short of it. There is no published trial of alfalfa meal as a compost starter, and no reason to think it does anything a nitrogen-rich material would not. Two or three handfuls into a stalled heap is plenty.

Store it dry and sealed. Wetted alfalfa heats and moulds, which is the same microbial process you are paying for in the soil, happening somewhere less useful. Wear a dust mask when you are spreading the meal.

Good, but not on its own

Alfalfa gives you nitrogen, potassium, calcium, trace elements and a serious load of carbon, slowly, from a plant-based material with no slaughterhouse by-products in it. It is modest on phosphorus, which suits established British garden soil that has usually had plenty already. It gives very little magnesium, which sometimes matters.

Judge it as a soil builder that also feeds, rather than as a feed that happens to be organic. That is the honest description and it is the one that holds up.

Alfalfa is one useful input, not the whole kit. If you want the wider set, shop all our organic fertilisers.

Where it needs help is speed and magnesium. A stronger nitrogen source covers the first, kieserite or polyhalite the second. For roses specifically, there is a longer piece on alfalfa for roses covering rates, timing and where the fifty-year rose society tradition actually came from.

The bag will tell you about the hormone. What you are buying is a slow, well-balanced load of nitrogen and potassium with calcium attached — nutrition that earns its keep without needing a year-later soil-carbon story the bagged meal has not proven. Which is worth having, and does not need the folklore.